This invention is directed to an apodized mirror optical system which includes a petal mirror which has individually adjustable petals for adjusting the distribution of the illumination intensity at the focal plane.
Electronic systems are being made more compact by employing printed circuit boards of continually more dense circuit arrangement. This permits the reduction in physical size of the electronic assembly. The exposure definition of photosensitive material has improved to permit the making of photo exposure masks of continually finer definition. In order to take advantage of the advances in masks, photosensitive material, etching and other arts, the optical exposure of the photosensitive resist must be proper. The exposure must be substantially uniform over the exposure area to provide substantially uniform photoresist development and consequent etching characteristics. The illumination provided for the exposure must be well collimated to provide a sharp exposure at the edge of each detail in the mask.
A previous optical system is disclosed in Thomas A. Richter U.S. Pat. No. 4,389,115. That optical system produces substantially uniform illumination at the exposure plane by employing an integrator mirror. That integrator mirror has a plurality of facets, but after the desired configuration of the integrator mirror is established, it is replicated as a single structural element. Dourte, et al., U.S. Pat. No. 4,195,913 describes that integrator mirror in more detail. Each facet of the integrator mirror is configured to deliver the beam over the entire exposure plane area.
It is desirable to employ an optical system which provides the desirable result of substantially uniform illumination at the exposure plane without the employment of an excessive number or of optically complicated parts.